1. Technical Field
The present invention relates to orthopedic surgery and in particular to the provision and implantation of a novel hip prosthesis. More particularly the present invention relates to a novel femoral neck hip prosthesis that is implanted into the femoral neck and is coupled to the calcar bone of the femoral neck.
2. Background Art
In the United States approximately 500,000 hip replacements are performed on patients with hip arthritis. As the population ages and increases the number of hip replacement procedures can be expected to increase by 10 percent per year over the next decade.
The majority of hip replacements have conventionally included the replacement of the proximal portion of the femur with an elongate, often curved shaft that extends into the medullary canal of the femur. This design typically places unnatural stresses on the femur, which lead to pain and the consequent curtailment of activity for the patient. In addition to imposing unnatural stresses on portions of the femur, the negative effects of stress shielding of the portions of the femoral bone that normally carry the load transfer can result in bone weakness, fractures, and the need for subsequent revisions. It is also possible that with conventional hip replacement surgery the resulting leg length can be changed from the natural length, which can induce further gait, balance, back, and other problems for the patient. Because of the inducement of additional problems including subsequent hip fractures, the current designs of hip prosthesis often have a useful life less than that of the patient. This is particularly a problem for active, young patients.
Young patients with hip disease or injury presently have two options for hip replacement surgery, Total Hip Replacement and Surface Replacement. Total Hip Replacement requires the placement of a femoral prosthesis into the femoral canal and replacement of the acetabulum. The femoral prosthesis is fixed into the intramedullary canal of the femur at a level below the trochanteric region. This area of the femur is known as the isthmus. The isthmus provides a reproducible area for locking of the implant; however, by intrusively violating the femoral canal the options for revision surgery at a future date are limited. If revision surgery is required, it usually involves implanting a longer femoral prosthesis into the canal, a technique that is more difficult and less reproducible. Total Hip Replacement requires conventional open surgical techniques with resultant blood loss, pain, and prolonged recovery periods. Younger patients are more likely to have failure of their primary replacement procedure due to the increased mechanical demand of their active lifestyle and from a longer life expectancy standpoint. Faced with the prospect of a first, a second, and possibly more revisions over their lifetime, the Total Hip Replacement for a young patient is not a good first option. The prospect of leg lengthening of an average of 1 centimeter in Total Hip Replacement techniques is also a poor prospect for the young, active patient.
The second option for young patients, Surface Replacement is a technique for providing a contoured cap implant on the surface of the femoral head. This technique avoids the instrumentation of the canal and thus is less invasive. Surface Replacement has been considered to be the best option for active, young patients. If this preferred option fails, the young patient can then be treated with a total hip replacement. Surface Replacement of the hip does involve certain difficulties and subsequent problems for the patient. The surgical implant, which overlays the head of the femur must be precisely placed. There is a 2 to 4 percent failure rate of Surface Replacement implants due to imprecise placement. There is also a 2 to 4 percent failure of the implant due to vascular problems with the underlying femoral head. The vascular problems typically lead to collapsing of the femoral head. U.S. Pat. No. 4,332,036 issued to Sutter et al. provides an example of prosthesis with a cap over the head of the femur. While preserving a good portion of the calcar bone and avoiding the negative effects of medullary instrumentation attendant to the Total Hip Replacement techniques, the cap implant of Sutter et al. is still subject to the Surface Replacement problems discussed above.
While conventional implants have addressed the articulating needs of the hip joint, they typically have not addressed, and more often have exacerbating the stress loading problems of the hip. The Total Hip Replacement techniques have typically imposed unnatural stresses on the shaft of the femur into which the femoral prosthesis is implanted. The calcar is the hips natural bone dense area on the medial neck of the femur. It begins beneath the femoral head and travels along the medial neck to the level of the isthmus. The body lays down new bone in certain areas as a natural response to stress. The load transfer stresses through the hip are naturally carried from the femoral head to the long shaft of the femur by the calcar bone. The standard technique of removal of the neck of the femur and the instrumentation of the femur through the isthmus portion effectively guts the calcar bone in these regions and thus positions the patient for subsequent bone weakness, fractures, and the need for revision surgery. An example of this invasive and calcar destructive technique is shown in U.S. Pat. No. 5,035,717 issued to Brooks. The femur is prepared to receive the implant by cutting through the neck of the femur and instrumenting a large portion of the remaining calcar to receive the stem of the implant. Having thus removed or damaged the majority of the load transferring calcar bone, the technique attempts to transfer the load through the hip by providing a collar that overlays the cut surface of the femur and serves only to apply the stress loading in an unnatural way. Even in techniques which involve only femoral head replacement, much of the calcar bone is lost in the surgical process and the replacement head is fixed to the bone by a lateral plate, sometimes called a Thrust Plate, or by cementing small diameter stems into the medullary canal of the neck or by securing the head to the neck of the femur by use of a securing rod or post that extends completely through the load transferring calcar and anchored to the exterior of the femur. An example of the latter technique is shown in U.S. Pat. No. 7,104,995 issued to Crofford. The implant and technique of Crofford requires that the core of the load transferring calcar be bored completely through to provide passage for a fixation prosthesis, a post, which rather than using the natural strength of the calcar bone attempts to replace it with the post that passes completely through the calcar and terminates with an anchor on the exterior lateral wall of the femur. None of the conventional hip replacement techniques utilize the natural load transferring calcar bone in the neck of the femur as a loading/fixation system for the replacement implant.
For these reasons there remains a need for a hip replacement implant and technique that not only reproduces the articulation aspects of the hip joint but also utilizes the natural load transferring calcar bone of the femur so as to provide a hip replacement that avoids the shortcomings of conventional hip replacement implants and techniques and thus reduces the likelihood of subsequent failures and required revisions.